Patent GB-865090-A (GB)
Title: Improvements relating to servo mechanism
Inventor: VOLES ROGER
IPC: G05B19/4105
Filed: 1956-07-19
Abstract: 865,090. Electric copying machines. ELECTRIC & MUSICAL INDUSTRIES Ltd. July 12, 1957 [July 19, 1956], No. 22384/56. Class 38 (6). [Also in Group XXXVI] In a servo system operated from recorded signals representing desired displacements, wherein a difference signal is obtained between the recorded signal and one representing the actual displacement produced by the servomotor, means are provided for controlling the rate of application of the recorded signals in accordance with the magnitude of the difference signal as this is reduced towards zero, so that errors in the displacement produced by the servomotor are kept less than a predetermined value. The invention is particularly applicable to the control of machine tools where the digital information on a punched tape is converted to a smoothly varying control signal by means of an interpolator, the means for controlling the rate of application of the signals then acting by controlling the rate of operation of the interpolator. In the Figure, binary code information as to the x and y co-ordinates of successive reference points along the required contour of a workpiece is sensed at 1 and passed to interpolators 4 and 5 which derive signals representing x, y, dx/dt and dy/dt. The x and y signals, after modification at 17 and 18 as described below, are passed to amplifiers 15, 16, where they are compared with displacement signals from the motor-driven potentiometers 19, 20 respectively. Any difference operates the appropriate servomotor 9 or 12 and shifts the worktable 8 until the difference signal becomes zero. The dx/dt and dy/dt outputs from the interpolators are fed to the stator windings 22, 23 of a resolver having quadrature rotor windings 24, 24a, as described in Specification 863,114. The servomotor 26 is thus energized from the amplifier 27 to set the shaft 25 to a position corresponding to the angle 0 of the normal to the required workpiece profile (Fig. 2, not shown). A further rotor winding 29 on the same shaft is supplied with a signal from the manually-operated potentiometer 28 representing r, the radius of the cutting tool. The resolver stator windings 30, 31 generate signals r cos # and r sin  #which are passed as correcting signals to the adding circuits 17, 18. A third resolver on the same shaft has stator windings 33, 32 supplied with the difference signals so that the signal induced in the rotor 34 represents ( #x cos 0 + #y sin #), i.e. the resolved part of the displacement errors in the direction of the normal to the contour of the workpiece at any instant. This signal is passed to the adding circuit 38 where it is combined with a further correction signal about to be described. The device 39 (e.g. a biased rectifier) transmits a signal representing the modulus of its input when the latter exceeds a predetermined value and drives the interpolator shaft 7 accordingly via the servo amplifier 40 and motor 6; the input 40a determines the normal speed of the motor. The output from the amplifier 27 (representing ##, the error in #) is squared at 35 and combined at 36 with a signal representing ¢r to give an input to 38 proportional to ¢r( ##)<SP>2</SP>. This compensates for errors in the setting of the shaft 25 and can be understood from Fig. 3 (not shown). The invention can also be applied to servo mechanisms producing displacements in three co-ordinate directions or to systems employing polar or cylindrical co-ordinates. Interpolators.-A suitable interpolator is described (see Group XXXVI) which is the same as that of Specification 807,643, modified to provide extra outputs representing dx/dt and dy/dt. The outputs can be further smoothed by the addition of a sub-interpolator as in Specification 796,994, or the latter can be used alone as the main interpolator. Specifications 830,462 and 849,091 also are referred to.